When a patient is anesthetized for a surgery or the like, it is required to maintain a suitable anesthetic depth so as to protect the patient from stress or pain due to the surgery. Conventionally, for example, an anesthetic depth has been subjectively or empirically recognized by monitoring a change of a blood pressure, a heart rate or a respiration rate of the patient, resulting from a stimulation of the surgery, or observing a reflex of eyelashes, a dimension of a pupil, a hue of a limb or a body temperature of the patient.
However, since the anesthetic depth recognition based on the change of the blood pressure, the heart rate or the respiration rate, or based on the reflex of the eyelashes, the dimension of the pupil, the hue of the limb or the body temperature is performed depending on a subjective judgement of anesthetists or the like, the anesthetists are required to have a long-time experience and a skill. In addition, it is not easy to objectively or accurately recognize the anesthetic depth. It is therefore an object of the present invention to provide an anesthetic depth measuring apparatus for objectively measuring an anesthetic depth of a living subject.
The inventors of the present invention have continued their study in the background of the above described situation, and they have found that a magnitude of a pulse period fluctuation component produced in synchronism with a respiration of a subject, a magnitude of a blood pressure fluctuation component having a frequency lower than a frequency of the respiration of the subject or a rate of change of the pulse period to the blood pressure of the subject has a close relation to an activity level of subject's parasympathetic nerve or sympathetic nerve, whereby an anesthetic depth of the subject can be objectively measured on the basis of the magnitude of the above mentioned fluctuation component, when the subject is anesthetized.
Moreover, the present inventors have found that the influences of the anesthesia on a peripheral body temperature and a deep body temperature are different from each other, and a difference between the peripheral body temperature and the deep body temperature has a close relation to an activity level of a nerve of the subject, whereby an anesthetic depth of the subject can be objectively measured on the basis of the difference. The present invention has been developed based on those findings.